Apparatus for storing liquefied gas near atmospheric pressure



June 29, 1965 J. B. MAHER ETAL 3,191,395

APPARATUS FOR STORING LIQUEFIED GAS NEAR ATMOSPHERIC PRESSURE Filed Julys1, 196s f f/f l/ ',f/ f t 7 @9 M 3i a e INVENTORS.

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United States Patent O ice 3,191,395 APPARATUS FOR STORING LIQUEFIED GASNEAR ATMOSPHERIC PRESSURE James B. Maher and .lames Mair, Chicago, Ill.,assignors to Chicago Bridge & Iron Company, Hinsdale, Ill., acorporation of Illinois Filed July 31, 1963, Ser. No. 298,867

1 Claim. (Cl. 6ZS4) This invention relates to the storage of liquefiedgases. More particularly, this invention is concerned with novelapparatus and methods for regulating the vapor pressure in storage tankscontaining liquefied gases.

Large quantities of liquefied gases are stored in insulated storagetanks. A large high capacity refrigeration system is provided to convertthe filling stream of gas to a liquid, or to cool awarm liquid stream,before itis.

pumped into the tank. Normally a second, smaller refrigeration system isused to compensate for heat-leak through the tank and maintain the gasin the liquefied condition.

Gases which are vapors at normal temperatures and pressures are storedin this way, including the hydrocarbons containing from 1 to 4 carbonssuch as methane, ethane, propane, isopropane and butane, as well asother gases including ammonia, carbon dioxide and oxygen.

It is common practice in most storage facilities to hold the liquefiedgas almost exactly at a temperature which in theory is calculated toexert the desired vapor pressure in the tank. For example, if the tankis designed for 1.0 p.s.i.g. then the liquefield product is normallyheldV at a temperature which gives a vapor pressure of about 0.5p.s.i.g.

Itis also customary to include means ancillary to the storage tank forproviding either replacement vapor to the vapor space of the storagetank during periods of pump-out or a rise in barometric pressure, or forreceiving expelled vapor when the pressure builds up, to maintain afairly uniform vapor pressure in the tank.

In order to prevent the build-up of excessive vapor pressure in thetank, it is the practice to withdraw gas vapor from the tank and liquefyit through the holding refrigeration system and then return it to thestorage tank.`

Such a system involves a continuous starting and stopping of therefrigeration unit and this, in itself, is costly and undesirable andalso involves the investment in, and maintenance of, a secondrefrigeration system.

' Although it has ben assumed in the past that the pressure in the vaporspace in the storage tank would be determined by the temperature of theliquefied gas stored in the tank, it has been found that the pressure inthe vapor space is greatly affected and substantially determined by theheat-leak through the unwetted portion of the storage tank above theliquid, and that the vapor is often at a condition of super-heatregardless of the temperature of the stored liquid, provided, of course,that the temperature is low enough to prevent boiling of the liquefiedgas.

Because in practice the vapor pressure in the tank is not determined bythe temperature of the liquefied gas, but rather is determined by theheat-leak in the unwetted portion of the vessel, the liquefied gas canbe stored at a sub-cooled temperature without subjecting the tank to avacuum through a reduction `in vapor pressure which might cause itscollapse. The vapor in a tank containing subcooled liquefied gas will bein a super-heated condition because of the heat-leakage. As a result,the pressure of the vapor will be considerably higher than the pressurewould be if determined by the temperature of the subcooled liquefiedgas. Because a liquefied gas can be stored in such tanks at subcooledtemperatures, it is practical to store refrigeration in the productitself by 3,191,395 Patented June 29, 1965 duce the vapor pressuretherein or the withdrawn subcooled liquefied gas can be vaporized andthe resulting vapor conducted to the vapor space of the tank to raisethe vapor pressure therein.

More particularly, there is provided by this invention a method ofcontrolling the vapor pressure in an enclosed, insulated storage tankpartially filled with a subcooled liquefied gas which comprisesdetermining the vapor pressure in the tank and its difference from apreselected pressure range, withdrawing subcooled liquefied gas from thetank and,`when the vapor pressure in the tank is above the preselectedpressure range, delivering the liquefied gas to the vapor space in thetank to condense some of the/vapor and thereby lower the pressure and,when the vapor pressure in the tank is below the preselected pressurerange, vaporizing the withdrawn liquefied gas and delivering the gasvapor so formed into the vapor space in the tank to raise the vaporpressure therein to the preselected pressure range, and discontinuingWithdrawal of liquefied gas when the vapor pressure in the tank is inthe preselected pressure range.

The invention will now be described in conjunction with the attacheddrawings which show the process, and novel combinations of apparatususeful in practicing the process.

FIG. l shows in schematic form a storage facility for liquefied gas anda novel combination of apparatus for The drawing of FIG. 1 shows aninsulated storage tankA 1 for holding liquefied gas 2 at a subcooledtemperature. The tank is designed to be partially filled, with a vaporspace in the upper part 3. A refrigeration plant 4 is provided havingcooling coils 5. The inlet conduit 6 supplies gas, or already liquefiedgas, to the refrigerating coils 5 from which liquefied gas emerges andis conveyed by conduit pipe 7 into the storage tank 1. The liquefied gasmay be cooled by the refrigerating coils to a pressure corresponding tothe atmospheric pressure storage temperature, such as 0.5 p.s.i.g., orit may be immediately subcooled. The refrigeration system need only beof sufficient capacity and horsepower to bring the incoming stream downto atmospheric pressure storage temperature after which the same systemcan be used to cool the liquefied gas down to a subcooled temperature.

T obring and maintain the liquefied gas at a subcooled tem-perature,there is provided withdrawal conduit pipe 8 for removing liquefied gasfrom the storage tank by means of pump 9 from which the liquefied gas isconveyed by conduit 10 to the refrigeration coils 5 where it issubcooled and returned to the storage tank by conduit pipe 11. Suitableinsulation is provided on all equipment and pipes described or formingpart of the system as is warranted to maintain the product at thetemperature necessary or desirable. A spray ring 12 is provided insidethe storage tank for feeding the subcooled liquefied gas from conduit 11substantially throughout the mass of liquefied gas in the tank.

The combination of apparatus used to control the vapor pressure in thevapor space 3 of the storage tank by use a of the subcooled liquefiedgas 2 in the tank in Ypart comprises means for withdrawing subcooledliquefied kgas from the tank such as conduit kpipe 13 which communi-Cates with pump 14 for facilitating removal of the liquefied gas. Theliquefied gas is then conveyed in a conduit pipe 15 to a three-way valve16. The valve 16 is -responsive to, and actuated by, pressure controller17 which in turn is responsive to the vapor pressure in the vapor spaceof the tank. The pressure gauge 17 actuates the three-way valve 16 byany suitable pneumatic or electrical means 25.

Communicating with the valve 16 is conduit pipe 18 which is capable ofconveying liquefied` subcooled gas to a conduit pipe 19 which deliversthe liquefied gas into the vapor space 3 of the tank. Conduit 19desirably communicates with a spray ring 20in the tank for uniformlydistributing the liquefied gas into the vapor space.

Also communicating with valve 16 is conduit pipe 21 which deliversliquefied `gas to vaporizer 22. As shown in the drawing, the Vvaporizercomprises a heat exchanger over which air is circulated by means'of fan23 to vaporize the liquefied gas in the heat exchanger. After the gashas been vaporized, it is conveyed by conduit 24 into conduit pipe 19and then into the vapor space of the tank. The

vapor can also be distributed into the vapor space byY means of sprayring 20.

The pressure controller 17 is set at a preselected pressure rangefwhichcompletely shuts a rvalve 16 and thereby closes access to conduits 18and 21 when the vapor pressure in the tank is in a preselected pressurerange. When valve 16 is completely closed, pump means 14 is off. Whenthe vapor pressure in the tank falls below the preselected pressurerange, pressure controller 17 actuates valve 16 to open selectively sothat liquefied gas can pass into conduit 21 while maintaining the accessport to conduit 1S closed. The subcooled liquefied gas is then conveyedfrom conduit 21 to the vaporizer 22 and the vapor therefrom is conveyedinto the vapor space to raise the pressure in the tank.

An increase in the vapor pressure in the tank above the preselectedpressure range causes pressure controller 17 to actuate valve 16 to openselectively the port communicating with conduit 18 so that liquefied gascan bel routed into VYit while simultaneously keeping the access port ofthe valve leading to- Vconduit 21 closed. As a result, subcooledliquefied gas is conveyed from conduit 15 through the valve to conduit18 and thence into conduit 19, for delivery to the vapor space in thetank. The introduction ofthe subcooled liquefied gas` into the vaporspace condenses'the vapor therein and thereby lowersV the vaporpressure.

The system shown in FIGURE k2 is similar to that of FIGURE 1 but themeans for supplying liquefied gas or gas vapor to .thev vapor space ofthe tank are separate of eachl other. Those parts of the apparatus inFIGURE 2 which are common to FIGURE 1 have been numbered identically.

Conduit in FIG. 2 withdraws subcooled liquefied gas from the tankr bymeans of pump 31. It is conveyed by conduit 29 through valve 40 toconduit pipev 33 and then to vaporizer 22. The resulting vapor isconveyed by conduit pipe 34 to the vapor space 3 of the storage tank.Pressure controller 35 is responsive to the gauge vapor pressure in thetank and, by electric or pneumatic meansA 42, it actuates valve tosupply liquefied gas to the vaporizer whenjvapor is` needed yto raisethe pressure in the tank. Pump 31 is controlled to operate when valvev40 is open and to be oli when the valve is closed. The.

resulting vapor is conveyed by conduit 34 into the tank. No spray ringis necessary for vapor. Y

To decrease the pressure inthe vaporrspace of the tank,` subcooledliquefied gas can be withdrawn in conduit pipe l 36 by means of pump 37and conveyed by conduitrZS through valve 41 to conduit 38 and then intothe vapor space 3 of the tank. Pressure controller 39 is responsive to apreselected vapor pressure in the tank and it actuates, by electric orpneumatic means 43, valve 41 to open the same when the vapor pressure inthe tank goes above the preselected level. subcooled liquefied gas isthen conveyed by conduit 38 to the vapor space of the tank. If

- desired, it can be sprayed in by spray ring 44. Of course,

refrigeration means are provided with the apparatus of FIG. 2 similar tothat shown in FIG. 1 to maintain the liquefied gas in the storage tankat a subcooled temperature.

During the filling operation the vapor pressure in the tank can becontrolled by withdrawing subcooled liquefied gas and spraying it intothe vapor space to condense the vapors and thereby lower the pressure.

Since rapid withdrawal of liquefied gas can lead to excessive loweringof the vapor pressure in the tank, the vapor pressure can be regulatedby the described invention by withdrawing subcooled liquefied gas,vaporizing it and feeding the va-por into the vapor space of the tank.

Various changes and modifications of the invention can be made and, tothe extent that such variations incorporate the spirit of thisinvention, they are intended to be included within the scope of theappended claim.

I What is claimed is:

Apparatus comprising an enclosed insulated storage tank capable ofholding subcooled liquefied gas in the tank with a vapor space above thelevel of the liquefied gas, a pressure controller communicating with,and responsive to, variations in pressure of the gas vapor in the tank,means for pumping subcooled liquefied gas from the tank to adistribution valve responsive to, and actuated by, the pressurecontroller, first conduit means communicating with thevalve and thevapor space of the tank for delivering liquefied gas from the valve tothe vapor space in the tank when the vapor pressureis higher than apreselected pressure, a second conduit means communicating with thevalve and a vaporizer for vaporizing the liquefied gas, and a conduitmeans leading from the vaporizer to the vapor space in the tank fordelivering vaporized kgas thereto from the vaporizer when the vaporpressure in the tank -is lower than a preselected pressure, access tothe first conduit being selectively closable and open to the secondconduit by the valve in response to the pressure gauge when the vaporpressure in the tank reaches a preselected minimum pressure and gasvapor is to be introduced therein, and access to the second conduitbeing selectively closable and open to the first conduit by the valve inresponse to the pressure gauge when the vapor pressure in the tankreaches a preselected maximum pressure, said valve further being closedto both the first and second conduits when the vapor pressure in thetank is within a preselected range between the said preselected maximumand minimum pressures.

References Cited by the Examiner UNITED STATES PATENTS 1,963,922 6/ 34Robinson 62-54 2,790,307 4/57 Ayres r 62--54 2,943,454 7/ 60 Lewis 62-522,993,344 7/61 Reed 62-55 3,045,437 7/62 Aronson 62-54 3,108,447 10/63Maher et al. 6.2--54 FOREIGN PATENTS 685,425 11/ 39 Germany.

ROBERT A. OLEARY, Primary Examiner.

